Electrochemical process for conversion of biodiesel to aviation fuels

ABSTRACT

Methods for the conversion of a biofuel such as biodiesel into an alkane composition such as an aviation fuel, kerosine, or liquified petroleum gas product involve a series of electrochemical reactions. The reactions include oxidation of methanol to carbon dioxide, reduction of fatty acid esters, and cleavage of fatty acid chains at C═C double bonds. The methods are carried out by systems of two or more electrochemical reactors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No. 61/367,092 filed Jul. 23, 2010 and entitled, CONVERSION OF BIODIESEL TO AVIATION FUELS, the whole of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Fossil fuels are being replaced by renewable and sustainable energy sources such as biodiesel, biokerosine, and ethanol, which can supply the needs of many types of ground transportation by fueling cars, trucks, and rail. Nevertheless, many biofuels such as biodiesel are not suitable for use in cold climates because they contain long chain hydrocarbons that can solidify at operating temperatures in such environments. Biofuels are also not suitable for use as aviation fuel, both because of their solidification at the low temperatures encountered at high altitutde and because they lack the high energy density required of an aviation fuel.

There remains a need to develop methods to convert biofuels into other useful fuel mixtures, such as mixtures of short chain alkanes that can be used for ground transportation in cold climates and for aviation fuel.

SUMMARY OF THE INVENTION

The invention provides a series of electrochemical processes carried out by directed electrochemical reactors for use in improving the quality of a biofuel, converting a biodiesel fuel to a fuel containing short chain alkanes, and converting the fatty acid methyl esters of a biodiesel fuel into a mixture of aliphatic hydrocarbons. The reactors can be used individually or combined into systems to modify a biofuel for new uses, such as in aviation or motor vehicles and as a replacement for liquified petroleum gas in heating and cooking.

One aspect of the invention is an electrochemical reactor for cleaning up biodiesel via selective oxidation of alcohols and glycerol, and thereby producing cogenerated power,

Another aspect of the invention is an electrochemical reactor for selective elimination of unsaturation and cleavage of C═C bonds in fuels with a high degree of unsaturation, such as biodiesel derived from rapeseed oil.

Yet another aspect of the invention is an electrochemical reactor for hydrogenation of ester moieties in a biodiesel fuel for the generation of aliphatic hydrocarbons.

Each of the abovementioned reactors utilizes on-line monitoring of fuel components by analytical methods such as high pressure liquid chromatography and mass spectrometry. The reactors also utilize selected electrocatalysts in conjunction with ion conducting membranes.

Still another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that reduces excess MeOH in a biodiesel source material to yield a first composition containing methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that fragments the methyl esters of aliphatic carboxylic acids of the first composition by carbon-carbon double bond cleavage to yield a second composition containing short chain methyl esters of aliphatic carboxylic acids. The system further includes a third electrochemical reactor that hydrogenates the methyl esters of the second composition to yield a third composition comprising alkanes.

Another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition containing short chain methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that performs a Kolbe reaction, whereby the aliphatic carboxylic acids of the first composition are decarboxylated to yield a second composition containing alkanes.

Still another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition containing short chain methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that hydrogenates the methyl esters of the first composition to yield a second composition comprising alkanes.

Yet another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition that contains fatty acid esters and methanol; reducing the amount of methanol by electrochemical oxidation of the methanol to carbon dioxide in a first electrochemical reactor; fragmenting fatty acid chains in a second electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a third electrochemical reactor to yield a composition comprising alkanes.

Another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition containing fatty acid esters and methanol; fragmenting the fatty acid chains in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; oxidizing the short chain aldehydes to short chain fatty acids; and performing a Kolbe reaction in a second electrochemical reactor, whereby the short chain fatty acids are decarboxylated to form alkanes.

Still another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition containing fatty acid esters and methanol; fragmenting the fatty acid chains in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a second electrochemical reactor to form alkanes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing an embodiment of a system for the conversion of biodiesel to an aviation grade bio-fuel using a series of three electrochemical reactors.

FIG. 2 shows the results from a chronoamperometry test using a Pt/C (E-TEK, 30%) catalyst, at 0.55 V vs. a reference hydrogen electrode (RHE) in 1M ethanol with 0 mM, 1 mM, and 3 mM polyvalent transition metal complex (lead acetate) and PtRuTM/C catalyst at 0.55 V in 1 M ethanol, Pt loading was 15 ug/cm². TM refers to a transition metal.

FIG. 3 shows the electrochemical reactions that take place in Reactor II of the system shown in FIG. 1. The reactions carry out the cleavage at C═C double bonds of unsaturated fatty acid methyl esters.

FIG. 4A shows a schematic representation of Reactor III of the system shown in FIG. 1, which carries out hydrogenation of a fatty acid methyl ester. FIG. 4B shows the reaction mechanism carried out in the reactor shown in FIG. 4A.

FIG. 5 shows a flow chart for an embodiment of a system for the conversion of a biofuel source containing free fatty acids, fatty acid esters, and tryglycerides into an alkane fuel using three electrochemical reactors.

FIG. 6 shows schematic representations of Reactors I-III from the biofuel conversion system shown in FIG. 5. Each reactor schematic depicts the components of the reactor and the electrochemical mechanism carried out by the reactor.

FIG. 7 shows an experimental system (H-cell) for carrying out an electrolysis reaction.

FIG. 8 shows NMR spectra for styrene and the resulting products formed after 5 hours' electrolysis and 18 hours electrolysis.

FIG. 9 shows the results of GC-MS analysis of electroreductive reaction of methyloctanoate showing the formation of octanol and octanoic acid as well as other products.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a series of directed electrochemical reactors useful for modifying biofuels in several ways. One type of reactor can be used to clean up biodiesel via selective oxidation of alcohols and glycerol, while simultaneously generating power. Another type of reactor can selectively eliminate unsaturation found in many biofuel sources, such as rapeseed oil, by cleavage of C═C bonds, yielding short chain hydrocarbons. A third type of reactor hydrogenates ester moieties and generates aliphatic hydrocarbons from fatty acid esters. Each of these reactors is designed to perform specific tasks and can be outfitted with on-line monitoring of fuel components using known analytical tools such as high pressure liquid chromatography and mass spectrometry. The reactors utilize selected electrocatalysts in conjunction with ion conducting membranes, such as the perfluorinated sulfonic acid prototype represented by DuPont's Nafion® series. The electrochemical reactions of the invention can be integrated into existing biodiesel production technology or can be added to it as a post-production refining process, e.g., to convert biodiesel to aviation fuel.

As used herein, a biofuel is a hydrocarbon-based mixture obtained from plant, animal, or microbial sources which can be used as a fuel in internal combustion engines, jet engines, for heating or cooking, or for generating other forms of energy such as electricity. One type of biofuel is biodiesel, which is a mixture of mono-alkyl esters of long chain fatty acids. Biodiesel is typically produced by transesterification of fats using methanol as the alcohol, but other short chain alcohols such as ethanol, (iso)propanol, or butanol also can be used. Most vegetable oil sources of biodiesel contain fatty acids having a chain length from about 16 to about 22 carbons in length and from 0 to 3 C═C double bonds, most commonly 18 carbons in length and 1 or 2 C═C double bonds. The methods according to the present invention can convert such mono-alkyl esters into short chain alkanes having, for example, from about 1 to about 16 carbon atoms in length, or from about 2 to about 12 carbon atoms in length, or from about 3 to about 10 carbon atoms in length. The short chain alkane product can include straight chain unbranched alkanes, but also can contain branched chain, cyclic, unsaturated, or aromatic hydrocarbons. In certain embodiments, the short chain alkane product of the present invention is essentially free of aromatic hydrocarbons, or is essentially free of unsaturated hydrocarbons, or is essentially free of mono-alkyl esters of long chain fatty acids, or is essentially free of alkanes having a chain length of 12 or more, 14 or more, 15 or more, 16 or more, 17 or more, 18 or more, 20 or more, or 22 or more carbon atoms. In some embodiments, the short chain alkane product consists essentially of linear alkanes having a chain length of about 8-11 carbon atoms.

FIG. 1 depicts an embodiment of a system (4) for electrochemical conversion of a biofuel. The system employs three electrochemical reactors which operate in a coordinated fashion to convert esters of long chain unsaturated fatty acids to short chain alkanes. FIG. 1 shows the appropriate positions for each reactor in the system. Reactor I (1) carries out the selective electrooxidation of MeOH formed during biodiesel production. Reactor II (2) cleaves the fatty acid esters at C═C double bonds, resulting in shorter chains that reduce the tendency to form solids at low temperatures. Reactor III (3) reduces esters to alkanes, completing the conversion of biodiesel to a fuel suitable for aviation use.

In Reactor I, a polyvalent transition metal ion complex mediates the methanol oxidation process at the immobilized Pt catalyst active sites for anodic oxidation of methanol. To illustrate the mechanism, ethanol electro-oxidation on a Pt/C catalyst was studied in the presence of millimolar quantity of a polyvalent transition metal ion complex (lead acetate) using chronoamperometry over a one hour period. The results are presented in FIG. 2, and show the significant increase in steady state currents achieved in the presence of the polyvalent transition metal ion complex. The polyvalent transition metal mediation system functions for the electrooxidation of MeOH as well. Thus, in this first embodiment of a system and method for converting biodiesel to an alkane composition, Reactor I performs the electrooxidation of MeOH to CO₂ in order to reduce the amount of excess MeOH present in a crude biodiesel source composition.

In Reactor II, methyl esters of fatty acids derived from inedible sources, such as rapeseed oil, whose primary constituent is erucic acid, are cleaved at C═C double bonds by electrocatalytic oxidation with oxygen in the presence of Cu catalysts (homogenous mediated catalysis). The Cu²⁺/Cu⁺ redox couple has been applied to a C—C double bond cleavage reaction of styrene [1]. In E-reactor II of FIG. 3, the methyl ester is first oxidized by Cu²⁺ to produce a radical cation. This radical cation electro-migrates to the cathode compartment through a cation exchange membrane and reacts with molecular oxygen to form an oxethyl intermediate species. Two fragments with carbon chain length appropriate for aviation fuel are produced after decomposition of the intermediate species. While rhodium-catalyzed carbon-carbon double-bond cleavage [6] and cobalt(I1)-catalyzed oxidative cleavage of a carbon-carbon double bond [7] also could be used, the Cu2+/Cu+ redox couple is preferred because the previously used catalysts are expensive due to the value of precious metals or their complicated synthesis process.

In Reactor III methyl esters are reduced to alkanes. See, e.g., FIG. 4A and FIG. 4B. Two solid polymer electrolyte (SPE) reactors in series are utilized for conversion of fragments coming out from Reactor II to alkanes (suitable as aviation fuel) at moderate temperature and atmospheric pressure. The reactor can be assembled with a RuO₂ powder anode (62) and a carbon paper cathode (72) painted with catalysts (60, 70) that were hot pressed onto the opposing surfaces of a Nafion cation-exchange membrane (40) [2]. Water (10) is pumped (12) into the RuO₂ anode of the reactor and electrolyzed to O₂ (31) and H⁺ under constant applied current (50) (typically 0.10 A/cm²). As-produced protons can migrate through a Nafion membrane forced by an applied electric field and react with fuels (20) circulated (22) past the cathode to form reduced products (30). Two steps can be used for electroreduction of methyl esters to alkanes. (i) The methyl ester can be first electroreduced to the corresponding primary alcohol with catalysis by Mg on the anode, following scheme I [3].

(ii) The alcohol can then be transformed to an alkane product by an electroreductive method on a lead cathode [4], according to Scheme II.

Optionally, the number or capacity of Reactors III can be reduced if all or a portion of the methyl esters are converted to alkanes directly by catalysis.

Traditionally, the excess alcohol in raw biodiesel is removed with a flash evaporation process or by distillation. Such processes have several disadvantages, including energy consumption and product quality. Further, the flavor components resulting from decomposition of long chain organic compounds are contamination to the environment. Worse, a “burn on” effect may occur, resulting in highly viscous concentrates that tend to adhere to evaporator heat transfer surfaces [5].

The system of Reactors I-III described above has several advantages compared to traditional chemical reactions used for similar purpose. Conventional hydrogenation of esters is carried out under high hydrogen pressure and elevated temperature in slurry-phase or fixed-bed reactors. Such methods involve high energy consumption, low selectivy, and low product yield [8,9,10]. The simple mediator method used in the Reactor I needs only a low concentration of a transition metal complex (such as 0.5 mM lead acetate). In addition, the transition metal complex is not consumed and remains as a redox couple once the fuel cell runs. The mediator catalyst used in Reactor II is a common and inexpensive salt, CuCl₂. No reducing agents are required in Reactor II for the activation of molecular oxygen. Reactor III uses moderate temperature and atmospheric pressure conditions for hydrogenation of methyl esters. Proton donors can be generated by a highly efficient water electrolysis process. Specificity for the electroreduction of esters and alcohols to final alkane products is provided through the use of two specific and exclusive catalysts.

Fabrication and installation of the electrochemical reactors required by the invention are simple and convenient. The electrochemical catalytic systems are operated in an environmentally friendly manner, and there is no noise contamination produced. Conditions of mild temperature and relatively low pressure during the running of the reactors offer them long-term durability. The overall process offers high product yield and efficient energy conversion. The traditional refining process of biodiesel always has been highly energy intensive and has faced technical hurdles. The present system of three electrochemical reactors has the potential to overcome these problems and achieve the use of aviation biofuels as a commercially viable energy source, which will reduce emissions of NOx and unburned carbon dramatically and lower the cost of production of aviation fuel significantly.

The invention also contemplates other combinations of electrochemical reactors for the processing of biofuels. In one such embodiment, the cloud point of a liquid fuel mixture is reduced through a series of electrochemical reactions.

The lipid content harvested from a biological source such as a plant source or a microbial source such as algae or bacteria flows into an electrochemical processing cell where fatty acids and triglycerides are cleaved into lower molecular weight components. This cleavage occurs at carbon-carbon double bonds which, as the inventors have shown, can be preferentially broken through electrocatalytic oxidation. Once cleaved, the fragments can be used as liquid fuel. However, the properties of these fragments can lead to a high cloud point. To reduce the fuel cloud point, oxygen can be removed through a secondary electroreduction process. The inventors have demonstrated that methyl esters can be reduced to alcohols using an electrochemical cell with magnesium electrodes.

A series of directed electrochemical reactors enable higher efficiency and greater selectivity for conversion of bio-sourced oils such as those containing triglycerides, aliphatic free acids and esters into kerosene grade fuels. Specifically tailored electrochemical reactors eliminate unsaturation (cleavage of C═C bonds) in free acids, esters and triglycerides; followed by either hydrogenation of ester moieties for generation of aliphatic hydrocarbons or oxidation of free acids to alkanes via the Kolbe process.

The inventors are unaware of any viable process for conversion of bio-derived oils to kerosene-type alkanes in terms of energy balance, flexibility towards feedstock and modularity. Some of the steps in the system of the present invention, especially the selective cleavage of the unsaturation at C₈-C₁₀ part of the aliphatic backbone, have no analog in conventional catalytic routes. Conventional routes for bio-oil upgradation (conversion to alkane derivatives) typically involves hydro-deoxygenation [11]. However the conventional sulfide based hydrotreating catalysts contaminate products by incorporation of sulfur, deactivate rapidly by coke formation, and are potentially poisoned by trace amounts of water [12,13]. A new acid aqueous process based on select transition metal catalysts has been proposed for bio-derived compounds such as polyols [14], and limonene [15]. However these are largely aqueous processes where phase transfer catalysis has yet to be incorporated, and later expensive separation steps are invariably needed.

The entirely electrochemical process described herein allows for complete flexibility and smooth transition between reactors without expensive cleanup steps. It also provides for complete elimination of all oxygenated moieties from the derived bio-oil fragments to yield alkanes suitable for use as fuel.

As shown in the overall schematic of the process (FIG. 5), the process (400) involves the following steps:

-   -   (a) Carbon double bond cleavage: This is a common step         regardless of choice of feedstock (free acids or esters). In         case of triglyceride starting material, initial one pot         conversion to aliphatic methyl esters can be conducted using         conventional trans-esterification reactions. E-reactor 1 (100),         as described herein is designed to efficiently convert C₁₆₋₂₂         aliphatic oils with terminal ester or acid groups to C₈₋₁₁         aliphatic fractions with terminal acid or ester groups in         conjunction with new terminal aldehydes produced in the process.         This simple 2 electron (1.3 V) process is far more efficient as         compared to any catalytic cleavage reaction due to room         temperature operation, higher efficiency (turnover frequency,         TOF) and most important, selectivity. No conventional catalytic         process enables such a high order of selectivity. This is         important considering the likelihood of presence of unsaturation         in other parts of the feedstock.     -   (b) Depending on whether the cleavage process results in         terminal ends containing ester or acid, two separate process         paths are proposed.         -   PATH 1 in case of the presence of free acid would first             entail (a) conversion of terminal aldehyde groups obtained             in step 1 to acid via low temperature catalytic oxidation             and (b) a Kolbe process for conversion of the terminal acid             groups to alkane.         -   PATH 2 can be used for fractions containing ester terminal             groups. As a first step, the aldehyde terminal groups             obtained in step 1 can be converted to esters via a             conventional esterification reaction. This is followed by a             special electrochemical reactor for conversion of ester             moieties to alcohol (electro-reductive process) and then             finally to alkanes.

An entirely electrochemical system of processing a biofuel according to this embodiment has a number of highly desirable characteristics and advantages over non-electrochemical approaches:

-   -   (a) Electrode processes generally do not require the addition of         consumable chemical components other than the base feedstock.     -   (b) Electrode potential can be chosen based on the composition         of the feedstock, hence providing for a flexible system,         modifiable based on feedstock.     -   (c) Efficiency can be gained by coupling appropriate anodic         chemistry with select cathodic chemistry. This is shown for the         individual description of each reactor process.     -   (d) Electrochemical cells can be designed to operate         economically through a wide range of reaction scales.

In order to develop each of the reactor components, model compounds can be used to mimic the real bio-oil feedstock input. For the process described in FIG. 6 involving E-reactor 1 and route 1, erucic acid can be used. The alternative process involving E-reactor 1 and route 2 can use the ester form of erucic acid as the starting material.

E-Reactor I (100) carries out C═C bond cleavage. The C═C double bond cleavage reaction of the aliphatic backbone in bio-derived ester or acid (such as in the case of erucic acid or ester) can be promoted via electrocatalytic oxidation with oxygen in the presence of Cu-catalysts (115) (homogenous mediated catalysis) in the anode compartment (118). The Cu²⁺/Cu⁺ redox couple concept was applied in a C═C double bond cleavage reaction of styrene [16]. As shown for E-reactor I in FIGS. 3 and 6, the methyl ester of erucic acid is first oxidized by Cu²⁺ to produce a radical cation. This radical cation then undergoes a mediated charge transfer with oxygen (116) to create an oxethyl intermediate species and Cu⁺. The Cu⁺ on electro-migration to the cathode compartment (119) through a cation exchange membrane (130) (such as Nafion® from Dupont) undergoes redox transformation to Cu²⁺ which on reverse electromigration to the anode compartment completes the circuit. Two fragments with relevant carbon chain length for aviation needs can be produced after decomposition of the intermediate species. Structure 110 is the anode, structure 120 is the cathode, structure 150 is the power supply, and structures 135 and 140 are fluid inlets and outlets, respectively.

E-reactor 1 can be followed with catalytic oxidation of aldehyde moieties to either acid or ester terminal groups. In the case of conversion to acid terminal ends, this can be conducted using conventional Tollen's reagent (Ag catalyst in conjunction with NH₃). For conversion of the fractions containing ester terminal groups (such as those obtained from erucic ester) the aldehyde end groups obtained in step 1 can first be converted to ester groups using conventional base catalyzed esterification in conjunction with methanol. Both conversion steps have greater than 95% yield and typically use an 85-95° C. single pot process followed with phase separation between aqueous and oil phases. These steps are conventional.

E-reactor 2 (200) carries out Route 1 (Kolbe Process), the direct conversion of acids to alkanes. A decarboxylative dimerisation known as the “Kolbe Reaction” proceeds via a radical reaction mechanism, yielding aliphatic products directly [17,18]. Generally, the reaction can be represented as:

R₁COO⁻+R₂COO⁻→R₁-R₂+2CO₂   (1)

This reaction can be carried out using metered amounts of acetic acid. R₁ can be a methyl group, and R₂ can be a fragment, with two terminal acid groups obtained from previous step. However, if R₁ and R₂ are different, then alkanes R₁-R₁ and R₂-R₂ are also likely products. Although use of the Kolbe process has the potential of saving a lot of effort in conversion of the acid to alkane products, there remains the potential problem of making a mixture of alkanes with various lengths of C chains, causing difficulty for separation and purification. Process design can utilize a proton exchange membrane (240) [18]. The important consideration is using metered amounts of H₂O (270) in the cathode (220) compartment for H₂ evolution. This careful metering of H₂O is important, as the electro-osmotic drag in this reaction is towards the cathode and hence has the added advantage of removing excess water from the oils at the anode (210). In addition the use of Pt/C catalyst is mandated due to the use of proton exchange membranes. However, the catalyst loading can be kept low using careful membrane electrode assembly strategies. The final design consideration is to prevent reaction of long aliphatic acid components obtained from cleavage of unsaturation in the starting long chain acid or ester (230) (erucic acid). Therefore, acetic acid (260) can be used as input for preferential reaction with individual aliphatic fractions obtained from E-reactor 1. Two important strategies can be used.

-   Strategy 1. Use of Pt alloy electrocatalysts in conjunction with     first row transition elements, specifically Ni, Co and Cr. Presence     of more oxidizable component in close proximity to Pt has a strong     potential of promoting free radical initiated reaction with acetic     acid component instead of its longer chain analog. -   Strategy 2. Use of crystalline multiphasic chalcogenides such as     Rh_(x)S_(y), where specific atomic length scales can provide the     appropriate preferred bonding environment for promoting reaction of     longer chain aliphatic components with acetic acid based radicals.     Rh_(x)S_(y) is a stable electrocatalyst in conjunction with proton     exchange membranes.

In strategy 1, catalysis on a nanocluster surface can be used with different oxidative sites (Pt and alloying element), while in the second strategy individual elements can be used as catalytic sites in a well coordinated crystalline environment.

E-Reactor 3 (300) carries out Route 2, the conversion of methyl esters to aviation fuels of alkanes. Route 2, shown in FIGS. 4 and 6, employs a two level electrochemical reactor arrangement which is specifically designed to deoxygenate the ester containing aliphatic components derived from E-reactor 1 to alkane type products. Two solid polymer electrolyte (SPE) reactors in series can be used for conversion of the C₈-C₁₁ aliphatic ester moieties to corresponding alkane form at moderate temperature and atmospheric pressure. The first reactor aims at converting ester groups to corresponding alcohols. The reactor is assembled with a RuO₂ powder anode (360) and a carbon black cathode (370) painted with magnesium catalysts (362, 372) that are hot-pressed onto the opposing surfaces of a Nafion cation-exchange membrane (340) [19]. Water (310) is pumped (322) into the RuO₂ anode compartment in the reactor and electrolyzed to O₂ and H⁺ under constant applied current (350) (such as about 0.10 A/cm² and 1.5 V); see FIGS. 4 and 6. Proton migration through the cation exchange membrane (Nafion®, Dupont) to the cathode electrode compartment enables the reduction of the ester (320) moieties to form analogous reduced products (330). The electroreduction of aliphatic esters on a magnesium electrode is relatively inexpensive due to the orders of magnitude milder conditions required. The alternative process involves high pressure and temperature catalytic reduction using precious metal catalysts. As fundamental transformation reactions, successful examples of reduction of ester and deoxygenating alcohol have been reported in the literature. Boechat and his coworkers developed a one-spot method for reduction of methyl aromatic esters to alcohols with high yield and low cost [20]. Moreover, in a classical Barton-McCombie deoxygenation reaction, the radical chemistry was employed to deoxygenate an alcohol and form an alkane product with a trimethylborane-water complex [21]. However, the selectivity of the reduction reactions in the above organic methods may not be satisfactory in an industrial process. As an alternative procedure, the electroreductive method can be adopted for selective transformation of esters to corresponding alcohols [22] and aliphatic alcohols to alkanes [23]. Any free acid species or fragment during these electrochemical processes can be converted to alkanes directly by Kolbe electrolysis [17]. The product is the corresponding primary alcohol when the reaction is run in the presence of a proton donor (schematic 1) [22]. The alcohol moieties can then be transformed to an alkane product by an electro-reductive method on a lead cathode (schematic 2) [23].

Three separate reactor designs can be used. In the first, the electrodes are separate from the ion conducting membrane. Though simple to implement, this rendition requires the greater use of free electrolyte, which presents problems later when purification of the fragments is conducted. Second is the use of half membrane electrode assemblies, wherein either the anode or the cathode would be bonded to the membrane. This method allows for the electrode where the primary process is occurring to be in a high throughput flow through mode. Third is the formulation of an anode and cathode bonded membrane electrode assembly (MEA). Here, electrolyte conductivity and that of the charged species is not a problem; however, proper mass transport of the reactants to the appropriate interfacial reaction zone has to be ensured.

Design of E-Reactor 1. The membrane electrode assembly can be designed using porous carbon electrodes (uncatalyzed) using various thicknesses of Nafion® (Dupont) membranes. Carbon electrodes with various levels of porosity can be used for correlating with mass transport measurement (electrochemical polarization measurements). Flow through plates can be designed using graphite plates for effective turnover numbers and charge transfer efficiencies. Both parameters can be measured under glavanostatic conditions with aid of an online GS-MS set up.

Design of E-Reactor 2. Membrane electrode assemblies can be designed with Pt/C electrodes on anode and cathode sides (e.g., 60% on C, 0.4 mg/cm² loading). The metering arrangement for cathodic hydrogen evolution reaction can be designed in keeping with water activity at the anode stream containing the aliphatic acid moieties.

Strategies can be implemented for promoting radical initiated reactions between aliphatic acid components obtained from E-reactor 1 and acetic acid derived radicals. This is intended to prevent formation of larger chain aliphatic chains due to reaction between the aliphatic components from E-reactor 1. Two approaches can be used.

Approach 1. Pt alloys with Cr, Ni and Co can be employed with the same electrode loadings as described above.

Approach 2. Use of crystalline chalcogenide catalyst, specifically Rh_(x)S_(y), prepared using a thiol based process, with the same loadings as described above.

The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.

Design of E-Reactor 3 Assembly. This includes two separate electrochemical cells. The first reactor (E-reactor 3.1) can be designed for conversion of the ester moieties to alcohol followed with a second tandem reactor (E-reactor 3.2) for its conversion to alkanes. The membrane electrode assembly for reactor 3.1 can comprise a RuO₂ anode and a Mg cathode. In this case, RuO₂ and Mg inks can be directly deposited into the Nafion membrane in conjunction with a solubilized form of the Nafion polymer. Since O₂ evolution is occurring at the anode electrode no carbon black is required on that electrode layer. This avoids difficulties in preventing carbon corrosion. At the cathode, Mg catalyst can be used in the same loading as prescribed above. Short periods of voltage reversal are can be used to keep the appropriate TOF's. This is from the perspective of cleaning the Mg electrode at a regular interval (every 30 s). Since voltage swings are mandated, no carbon is added to the Mg side of the electrode as well. Since O₂ evolution is involved in the first cell, no carbon plates are used. Instead the bi-polar plates in this reactor can be constructed with Ti flow through plates. The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.

Reactor 3.2 can be fitted with a Pb/C cathode for electroreduction of alcohol with an interpenetrating proton-conducting phase, typically a sulfonated polymer such as Nafion, and a RuO₂ anode electrode. These components transport electrons, protons and gas or solution phase reactants and products to and from the electrocatalyst site. The oxygen evolution anode can be similar to the one used in reactor 3.1. As mentioned earlier, this anode can be directly deposited on the membrane surface (Nafions®, Dupont) and does not contain any carbon black. Ti based bipolar plates can be used for E-reactor 3.2. This is due to oxygen evolution anode electrode being used as the proton source.

Oxygen from E-reactors 3.1 and 3.2 can be recirculated to E-Reactor 1.

The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.

Various techniques can be adopted for enhancing conversion efficiency. For example, the concentration of Cu ion catalyst can be optimized based on a balance of high performance and easy separation from the final products. Solvents with boiling point or viscosity different from the reactant (erucic acid) and the obtained fractions can be used to ensure convenient post-reaction separation.

In E-reactor 2 (Kolbe Process), careful control of the dissociation of acid moieties can be obtained via adjustments to solvent composition. Central to the success of the Kolbe process is promotion of the reaction between acetic acid and the aliphatic fractions obtained from E-reactor 1. Considering the mechanism of the Kolbe process, the reaction is initiated by the carboxylate ion undergoing an initial discharge to form an adsorbed radical. RCOO⁻→RCOO (ads)+e⁻. Steric considerations can be exploited via the use of Pt alloys and chalcogenide electrocatalysts. However this process can be further promoted by careful adjustment of the solvent conditions to promote dissociation of the acid.

Data have been collected to demonstrate the direct anodic cleavage of the styrene moiety to yield corresponding olefins. The electrolysis reaction was carried out in an H type cell (shown in FIG. 7) divided with a glass frit. Two acetonitile solutions containing 0.02M CuCl₂ and 0.1M styrene, respectively, were put into the cathode and anode sections of the cell. A 0.1M NaClO₄ solution was used as the supporting electrolyte. The anodic oxidation was conducted at 1.0 V vs. Ag and refluxed at 60 C for 16 hours under O₂ atmosphere. The products were identified with a 700 MHz 4-channel NMR spectrometer. FIG. 8 clearly shows that aldehyde groups were formed after the electrolysis reaction. The peaks for alehyde groups grew as a function of time, indicating accumulation of the oxidized products with C—C double bond breaking on styrene. The system also pertains to the electrooxidation of erucic acid due to the similar “chemical environment” of C═C double bond on styrene with that on erucic acid.

Conversion of methyl esters to alkanes proceeds by first transforming the methyl ester into an alcohol followed by a second electroconversion into alkane. The reduction of methyl 1-octanoate to octanol was demonstrated using an electrolysis process with a LiClO₄ electrolyte and t-BuOH as a proton source, with magnesium and silver electrodes. The products of this electroreduction were shown using a GC-MS technique. In addition to being quantitative, GC-MS also provides detailed structural information for an analyte as a result of diagnostic fragmentation patterns resulting from ionization of the molecule. FIG. 9 shows the result of GC-MS analysis using an Agilent 6890 GC with 5973 mass selective detector (30 m×0.25 mm HP-MS5 capillary column with the masses acquired from 50-500 Da) for the electroreduction of methyloctanoate. The region specifically related to the methyloctanoate is indicated although other products were detected during the analysis. The presence of underivatized octanoic acid can be corrected by adjusting the ratio of silylating reagent to crude reaction mixture to insure complete derivatization of all hydroxyl containing analytes.

While the present invention has been described in conjunction with certain preferred embodiments, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof.

REFERENCES

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1. A system for the chemical conversion of a biodiesel to an alkane composition, the system comprising: a first electrochemical reactor that reduces excess MeOH in a biodiesel source material to yield a first composition comprising methyl esters of aliphatic carboxylic acids; a second electrochemical reactor that fragments the methyl esters of aliphatic carboxylic acids of said first composition by carbon-carbon double bond cleavage to yield a second composition comprising short chain methyl esters of aliphatic carboxylic acids; and a third electrochemical reactor that hydrogenates the methyl esters of said second composition to yield a third composition comprising alkanes.
 2. A system for the chemical conversion of a biodiesel to an alkane composition, the system comprising: a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition comprising short chain methyl esters of aliphatic carboxylic acids; and a second electrochemical reactor that performs a Kolbe reaction, whereby the aliphatic carboxylic acids of said first composition are decarboxylated to yield a second composition comprising alkanes.
 3. A system for the chemical conversion of a biodiesel to an alkane composition, the system comprising: a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition comprising short chain methyl esters of aliphatic carboxylic acids; and a second electrochemical reactor that hydrogenates the methyl esters of said first composition to yield a second composition comprising alkanes.
 4. The system of any one of claims 1-3, wherein the carbon-carbon double bond fragmentation process is performed with the aid of a Cu²⁺/Cu⁺ redox couple.
 5. The system of any one of claims 1-3, wherein the electrochemical reactor that performs the carbon-carbon double bond fragmentation process comprises a cation exchange membrane.
 6. The system of any one of claim 1 or claim 3, wherein the electrochemical reactor that performs the hydrogenation process comprises a solid polymer electrolyte.
 7. The system of claim 1 or claim 3, wherein the hydrogenation process is performed in two sequential reactions by two solid polymer electrolyte reactors working in sequence, the first reaction converting esters or aldehydes to primary alcohols and the second reaction converting the primary alcohols to alkanes.
 8. The system of claim 1 or claim 3, wherein the electrochemical reactor that performs the hydrogenation process comprises a cation exchange membrane.
 9. A method for the chemical conversion of a biodiesel to an alkane composition, the method comprising the steps of: providing a crude biodiesel composition, the biodiesel composition comprising fatty acid esters and methanol; reducing the amount of methanol in the biodiesel composition by electrochemical oxidation of methanol to carbon dioxide in a first electrochemical reactor; fragmenting fatty acid chains of the biodiesel composition in a second electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a third electrochemical reactor to yield a composition comprising alkanes.
 10. A method for the chemical conversion of a biodiesel to an alkane composition, the method comprising the steps of: providing a crude biodiesel composition, the biodiesel composition comprising fatty acid esters and methanol; fragmenting fatty acid chains in the biodiesel composition in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; oxidizing the short chain aldehydes to short chain fatty acids; and performing a Kolbe reaction in a second electrochemical reactor, whereby the short chain fatty acids are decarboxylated to form alkanes.
 11. A method for the chemical conversion of a biodiesel to an alkane composition, the method comprising the steps of: providing a crude biodiesel composition, the biodiesel composition comprising fatty acid esters and methanol; fragmenting fatty acid chains in the biodiesel composition in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a second electrochemical reactor to form alkanes.
 12. The method of claim 11, wherein the step of hydrogenating comprises first the step of hydrogenating fatty acids to alcohols and subsequently the step of hydrogenating the alcohols to alkanes.
 13. The method of any one of claims 9-12, wherein the biodiesel source material comprises aliphatic chains of about 16 to about 22 carbon atoms in length.
 14. The method of any one of claims 9-12, wherein the final alkane composition comprises aliphatic chains of about 8 to about 11 carbon atoms in length.
 15. The method of any one of claims 10-12, further comprising the step of reducing the amount of methanol in the biodiesel composition by electrochemical oxidation of methanol to carbon dioxide. 